Demineralization of coal

ABSTRACT

The invention concerns a process for the preparation of demineralized coal, comprising the steps of: 
     (a) forming a slurry of coal particles, preferably at least 50% by weight of which particles have a maximum dimension of at least 0.5 mm, with an aqueous solution of an alkali, which solution has an alkali content of from 5 to 30% by weight, such that the slurry has an alkali solution to coal ratio on a weight basis of at least 1:1; 
     (b) maintaining the slurry at a temperature of from 150° to 300° C., preferably 170° C. to 230° C., for a period of from 2 to 20 minutes substantially under autogenous hydrothermal pressure and rapidly cooling the slurry to a temperature of less than 100° C.; 
     (c) separating the slurry into alkalized coal and a spent alkali leachant solution; 
     (d) regenerating the alkali leachant solution for reuse in step (a) above by the addition of calcium or magnesium oxide or hydroxide thereto to precipitate minerals therefrom; 
     (e) acidifying the alkalized coal by treatment with an aqueous solution of sulphuric or sulphurous acid to yield a slurry having a pH of from 0.5 to 1.5 and a conductivity of from 10,000 to 100,000 μs; 
     (f) separating the slurry into acidified coal and a spent acid leachant solution; and 
     (g) washing the acidified coal.

TECHNICAL FIELD

The present invention relates to a process for the preparation ofdemineralized coal and to demineralized coal produced by such a process.

BACKGROUND ART

Several methods have been described in the literature for producingdemineralized or low-ash coal for fuel and other industrialapplications, but none have achieved sustained commercial use. Improvedprocessing methods, based on a better understanding of the underlyingscience, are required in order to foster a successful adoption ofchemical cleaning methods for producing superclean coal and itsderivatives.

A process was developed in Germany during the 1940's for removingash-forming mineral matter from physically cleaned black coalconcentrates, involving heating the coal as a paste with aqueous alkalisolution, followed by solid/liquid separation, acid washing and waterwashing steps. Reports on this process (1,2) are the earliest accountsknown to us of a practical chemical demineralizing method to which theimprovements described here may be related. German practice showed thata demineralized coal with an ash yield of 0.28% could be produced from aphysically cleaned feed coal which had an ash yield of 0.8%.

The coal-alkali feed paste was stirred at 40°-50° C. for 30 minutes,then pumped through a heat exchanger to a continuously operablegas-heated tubular reactor in which the paste was exposed to atemperature of 250° C. for 20 minutes, under a pressure of 100-200atmospheres (10-20 MPa). The reaction mixture was then passed throughthe heat exchanger previously mentioned, in order to transfer heat tothe incoming feed, then cooled further in a water-cooled heat exchanger.

The cooled paste was diluted with softened water, then centrifuged toseparate and recover the alkaline solution and the alkalized coal. Thelatter was dispersed into 5% hydrochloric acid, then centrifuged torecover the acidified coal and spent acid, and redispersed in water. Thecoal was filtered from this slurry, dispersed again in another lot ofwater and centrifuged to recover the resulting low-ash coal as a dampsolid product.

American (3,4) and Indian (5-7) researchers used broadly similarchemical methods, with variations in processing details, to producelow-ash coals from other feed coals, most of which had much higherstarting ash levels than the coals that the Germans used. AnotherAmerican group (at Battelle) claimed advantages for:

(a) Mixed alkali leachants containing cations from at least one elementfrom Group IA and at least one element from Group IIA of the PeriodicTable (8,9).

(b) Filtration or centrifugation of the alkalized coal from the spentalkaline leachant, either at the reaction temperature or after rapidcooling to less than 100° C., in order to minimize the formation ofundesired constituents, presumably sodalite or similar compounds (9,10),and

(c) Application of the process to low-rank coals which dissolve in thealkali and which can be reprecipitated at a different pH from themineral matter, thus allowing separation and selective recovery (11).

Other researchers have studied scientific aspects of alkaline extractionof sulphur and minerals, including the relative merits of differentalkalis (12-14). Most American work has been directed more at theremoval of sulphur than metallic elements, and the acid treatment stepis often omitted. However, an American group (at Alcoa) has chemicallycleaned coal to less than 0.1% ash yield, concurrently achieving largereductions and low final concentrations of iron, silicon, aluminium,titanium, sodium and calcium. The aim was to produce very pure coalsuitable for conversion into electrode carbon for the aluminiumindustry. This was achieved by leaching powdered coal with hot aqueousalkaline solution under pressure (up to 300° C.), then successively withaqueous sulphuric acid and aqueous nitric acid at 70°-95° C. (15-16).

The present inventors' investigations have been conducted withAustralian coals, which usually contain less sulphur, but often moreash-forming mineral matter than Northern Hemisphere coals. For practicalindustrial applications, it would usually be necessary to start withfeed coals containing more mineral matter than the coal concentratesthat the Germans used, and to remove a larger proportion of it bychemical means so as to obtain products of similar purity.

Like the Germans, the present inventors find that sodium hydroxidesolution, unmixed with oxides or hydroxides of Group IIA cations, is anadequate alkaline leachant but they recommend using different alkaliconcentrations, coal/liquid ratios and leaching conditions. The presentinventors anticipate practical difficulties in separating alkalized coalfrom spent alkaline leachant on an industrial scale at the temperaturesand pressures used in the alkaline leaching step as claimed by Battelle(8,9), but acknowledge advantages in rapid cooling before separating thesolid and liquid components as claimed by Battelle (9,10) but previouslypracticed by the Germans (1,2). The present inventors recommend specificways of conducting the leaching, cooling and separating steps inassociation with other procedures.

Previous investigators have usually experienced difficulties inachieving very low ash levels, except when starting with clean coalconcentrates as feed. Having studied the chemical and physical factorsin more detail, the present inventors recommend specific methods andprocessing conditions, especially involving the acidification andwashing procedures, in order to minimize the residual mineral matterleft in the demineralized product. They have also found, contrary toexpectations and to German practice, that the process will demineralizecoarse batches (5-10 mm) to about the same extent and at about the samerate as with fine coal of typical pulverized fuel.

DISCLOSURE OF INVENTION

The present invention consists in a process for the preparation ofdemineralized coal, comprising the steps of:

(a) forming a slurry of coal particles, preferably at least 50% byweight of which particles have a maximum dimension of at least 0.5 mm,with aqueous solution of an alkali, which solution has an alkali contentof from 5 to 30% by weight, such that the slurry has an alkali solutionto coal ratio on a weight basis of at least 1:1,

(b) maintaining the slurry at a temperature of from 150° to 300° C.,preferably 170° C. to 230° C., for a period of from 2 to 20 minutessubstantially under autogenous hydrothermal pressure and rapidly coolingthe slurry to a temperature of less than 100° C.

(c) separating the slurry into alkalized coal and a spent alkalileachant solution,

(d) regenerating the alkali leachant solution for reuse in step (a)above by the addition of calcium or magnesium oxide or hydroxide theretoto precipitate minerals therefrom,

(e) acidifying the alkalized coal by treatment with an aqueous solutionof sulphuric or sulphurous acid to yield a slurry having a pH of from0.5 to 1.5 and a conductivity of from 10,000 to 100,000 μs,

(f) separating the slurry into acidified coal and a spent acid leachantsolution, and

(g) washing the acidified coal.

The improvements which are recommended for efficiently demineralizingblack coals to very low ash levels may be varied within the ambit of thepresent invention as appropriate to the circumstances and coal involved.These improvements are not limited in their application to Australiancoals but would apply to any other coal with similar characteristics,properties and composition.

In carrying out the process to the present invention, preferred reactionconditions as discussed hereunder are used:

(1) Selection of optimal conditions for the alkali leaching stage inorder to maximize dissolution of the mineral matter, to minimize attackon the organic matter, and to minimize the formation of insoluble sodiumaluminosilicates on the coal or within its pore structure. Theseconditions are best provided by

(a) Using a slurry or a paste, with an adequate quantity of water tofacilitate contact between the alkali and the minerals, and to removethe soluble reaction products and keep them in solution. A minimumliquid:solid ratio of 1:1 is recommended for convenience of stirring andtransferring, compared with the German practice of 0.4:1, with preferredratios ranging from 2:1 to 10:1, the higher ratios being preferred whenlarge amounts of minerals are to be removed. The leachant preferablycontains at least a small excess of alkali above the stoichiometricrequirements for dissolution of the minerals to be removed; the alkaliconcentration should be kept at the low end of the 5-30% practicalrange, preferably in the range of 5-20%, and most preferably in therange of 5-10%.

(b) Avoiding unnecessarily high temperatures. While temperatures of150°-300° C. are feasible, temperatures of 170°-230° C. are usuallyadequate to dissolve the commonest minerals, especially clays andquartz. Pyrolysis of the organic matter does not occur in thistemperature range, and chemical attack on the organic matter, forinstance at phenolic and carboxylic acid groups, is minimal for mediumto high rank coals. However, considerable dissolution occurs with lowrank coals, which are therefore less suitable for demineralization bythis process.

(c) Avoiding unnecessarily long and badly controlled heating. Shortresidence times of 5-10 minutes at the selected operating temperatureare preferred, with minimal heating-up and cooling-down times. Thisregime can be more easily provided either in a continuous reactor or inbatch autoclaves. Long residence times, and leisurely heating andcooling conditions, favor the unwanted side reactions which involveattack on the organic matter and formation of aluminosilicates. However,residence times of up to an hour or more are not excluded, and may beappropriate when low alkaline leaching temperatures are chosen.

(d) Using reasonably coarse coal particles instead of finely ground orpulverized coal. Slurries of coarse particles are easier to process anddewater than slurries of fine particles. Experiments have shown that theaqueous reagents penetrate coarse and fine particles equally well anddemineralization varies little with particle size.

(2) Procedures and equipment for conducting the alkaline leachingprocess may take several forms such as the following:

(a) A desirable procedure to minimize the occurrence of unwantedreactions during the heat-up period comprises heating a relativelyconcentrated alkali solution and an aqueous coal slurry separately tothe desired reaction temperature, then mixing them quickly andthoroughly before allowing the reaction time between them to continuefor the desired time. Experience with a small continuous reactor of thistype indicates that attack on the minerals is adequate, but attack onthe organic matter and formation of sodalite are minimized. In anotherpreferred embodiment of the invention, a previously heated alkalisolution is poured onto dry particulate coal.

(b) Suitable leaching reactors may comprise material including tubularconcurrent-flow reactors, stirred autoclaves operating batchwise, orwith continuous inflow and outflow, in single or multistageconfigurations, or countercurrent or crossflow systems.

(3) After the comparatively rapid dissolution of quartz and clays hasoccurred, the relatively slow formation and deposition of sodiumaluminosilicates (sodalites) begins to occur from solution. Thealkalized coal and spent leachant should preferably be separated quicklyafter leaving the reactor, in order to minimize contamination of theleached coal by sodalite. Alternative improvements to the standardprocess are then possible as follows:

(a) The spent leachant is mixed with sufficient calcium oxide or calciumhydroxide to precipitate the soluble silicate and aluminate ions astheir insoluble calcium salts, while simultaneously forming solublesodium hydroxide, thus regenerating the alkaline leachant for recycling.This procedure minimizes the amount of acid needed in the nextprocessing step and hence lowers the total cost of demineralizing thecoal. Instead of calcium oxide or hydroxide, the corresponding magnesiumsalts may be used, or the mixed oxides or hydroxides of calcium andmagnesium derived from dolomite may be used.

(b) Recovery of the sodalite by filtration or otherwise may provide avaluable byproduct, while reducing the amount of acid needed to completethe demineralization of the coal. Sodalite may be separated from thealkalized coal by physical methods such as selective screening, heavymedia float-sink methods, or froth flotation.

(4) When alkalized coal is acidified with a mineral acid, the sodalitepresent dissolves to form sodium and aluminium salts and silicic acid.Typically, after removal from the acid leachant, the demineralized coalstill gives an ash yield of 0.2-1.0%, and the predominant mineralcomponent in the ash is usually silica. Some of this silica may arisefrom the soluble silicates and silicic acid rather than from undissolvedquartz or siliceous plant material. Improvements to the process aretherefore directed at preventing the retention of silicates or theformation of silica gel in the product. This objective can be achievedby the following procedures used individually or in combination:

(a) The alkalized coal is acidified to a pH of about 1 as rapidly aspossible, so that the coal experiences only very transitory contact withsilicate solutions of near-neutral (pH 7) or strongly acidic (pH <<1)reactions, both of which favor formation of silica and alumina gels. Itis desirable to add the alkalized coal to an acidic solution ofsufficient concentration to ensure that the resulting mixture inmaintained as close as possible to pH 1, with rapid and thoroughagitation to ensure that this acidic environment is quickly establishedthroughout the porous structure of each particle. Acidification may becarried out batchwise or continuously using this principle.

(b) When the alkalized coal has been acidified, it should be separatedas soon as practical from the spent leachant and well washed, preferablyusing countercurrent techniques.

(c) To further discourage silica gel formation, and the trapping ofother minerals by silica in the pore structure of the coal particles,the acidified coal may be first washed with a fresh acid solution ofabout pH 1 to remove the relatively concentrated solutions of dissolvedminerals therefrom by the acid leaching. Optionally, an organic acidwith a sufficiently high dissociation constant, such as acetic acid, maybe used for this purpose in order to minimize the concentration ofinorganic anions remaining on or in the coal. Solutions of ammoniumsalts are also useful for washing out residual minerals. The finalwashing is carried out with water, which may be deionized by establishedmethods before use.

BRIEF DESCRIPTION OF THE DRAWING

Hereinafter given by way of example only is a preferred embodiment ofthe present invention described with reference to the accompanyingdrawings in which:

FIG. 1 is a flow sheet showing the steps of the process according to thepresent invention; and

FIG. 2 is a diagrammatic representation of laboratory apparatussimulating.

THE BEST MODE OF CARRYING OUT THE INVENTION EXAMPLE NO. 1

A 1 kg sample of Liddell Foybrook coal with an ash yield of 8.5%(particle size--200 um) was slurried with 2.5 L of water and stirred ina holding tank 10. A second solution of 20% w/w of NaOH was contained ina second tank 11. Both the coal slurry and caustic solution were pumpedseparately via metering pumps 12 and 13 at 3.5 and 25 liters/hr,respectively, and heated to 200° C. with electrical immersion heaters 14and 15 respectively. The two solutions were mixed in a 500 ml stainlesssteel pressure vessel 16 and the solution maintained at 200° C. for theduration of the slurry in the vessel, approximately 5 min. The alkalicoal slurry was rapidly cooled to room temperature and collected incontainer 15 after leaving the pressure relief valve 18.

The slurry was filtered on a Buchner funnel and washed with water toremove excess alkali. A small sample of the washed coal was dried andthe ash level determined by standard techniques. The ash yield, whichwas comprised of mainly sodalite, was 7.3%.

The filtrate was pale in color, and, after acidifying a 20 ml portion, aprecipitate was collected which represented <0.05% of the coal.

The remaining coal filter cake from the Buchner funnel was treated with0.1M sulphuric acid and maintained at pH of 1 with sufficient water togive a conductivity reading of 50,000 μS. The mixture was stirred for 45minutes then filtered and washed with distilled water until the filteredsolution had a conductivity of <10 μS. A sample of the coal was thendried and an ash yield determined. The demineralized Liddell coal had anash yield of 0.5%.

The bulk of the alkali liquor from the initial filtration was treatedwith 100 gm of lime Ca(OH)₂ and stirred for 2 hours, then filtered. Theliquor (still slightly colored) was analyzed for silicon content and if<200 ppm was used for subsequent leaching studies.

EXAMPLE NO. 2

A 100 gm sample of Liddell Foybrook coal, with an ash yield of 8.5%(particle size--200 μm) was slurried with 300 mls of 15% caustic sodasolution and placed in a 1 L stainless steel autoclave. The autoclavewas heated to 200° C. over 35 minutes then allowed to cool to 80° C.over 11/2 hours and the slurry then recovered from the autoclave. Afterfiltering the slurry in a Buchner filter the filtrate was darkly coloreddue to dissolved humic acids. The amount of humic acids was determinedby acidifying a 20 ml portion of the liquor and filtering to collect theprecipitated organics. After weighing the precipitate the percentage ofdissolved coal was calculated at 1%. This filtrate which containedmainly sodium silicate and excess caustic was treated with lime Ca(OH)₂and stirred for 2 hours. When the concentration of silicon in solutionhad dropped from the initial concentration of 2000 ppm to <200 ppm thelime treated slurry was filtered and the regenerated caustic solution(black liquor) was reused for further leaching studies. The alkalizedfilter cake coal after washing, to remove excess caustic was slurriedwith 200-250 ml water and acidified to pH of 1 with sulphuric acid.Conductivity of this solution measured 25,000 μS. After 45 minutes, thisslurry was filtered and washed with distilled water until theconductivity was <10 μS. The ash yield of this demineralized Liddellcoal was 0.60%.

EXAMPLE NO. 3

Example 2 was repeated using coal feed which had a particle sizedistribution of less than 3 mm with 50% of solids between 3 and 0.5 mmand 50% less than 0.5 mm. The coal filter cake after separation of thealkali solution was treated as in Example 2.

Five kilograms of coarse alkalized coal prepared as in the above methodwas found to have an ash yield of 11.3% (mostly sodalite). Frothflotation of this coal was performed in a conventional laboratory scaletest unit using diesel oil (0.1%) and methyl isobutyl carbinol (0.01%)frothing agent, and an air flow sufficient to give a good froth withoutexcess turbulence. The ash yield dropped from 11.3% to 6.3% ash.

A similar set of experiments was run to collect a quantity of coarsealkalized coal and several kilograms of the coarse alkalized coal waswashed in a countercurrent wash unit at the rate of 12 kg coal/hr washedwith 24 kg of water/hr. Under these conditions, fine underflow materialwas collected in the waste water, which was rich in sodalite, asindicated by the ash yield which was 73% sodalite.

These two steps are important in that they recover sodalite-richconcentrates and reduce the quantity of acid necessary for subsequentacidification of the coal.

The details of the process and the importance of the respective processparameters essential to this invention may be better understood byreference to the following examples drawn from extensive laboratory andsmall rig studies.

EXAMPLE NO. 4

Experiments showed that a caustic coal paste is as efficient as adiluted solution for removing the mineral matter from coal, providedsufficient caustic soda is present. Sufficient water should be providedto achieve adequate stirring and transportation and transfer ofmaterial, preferably a 30% slurry. In practice, a maximum slurryconcentration of 50% has been found.

Ash removal from a Vaux steam coal treated at 200° C. under thefollowing conditions is shown below:

    ______________________________________                                                 Residence time                                                                          Slurry      % Mineral                                               at 200° C.                                                                       Concentration                                                                             Removed                                        ______________________________________                                        Vaux fine coal                                                                           2 min.      70          48                                         (floats containing     35          46                                         2.4% ash)                                                                     Vaux fine coal                                                                           2 min.      70          83                                         (sinks containing      35          84                                         14.2% ash)                                                                    Vaux coarse coal                                                                         30 min.     70          58                                         (floats containing     35          54                                         2.4% ash)                                                                     Vaux coarse coal                                                                         30 min.     70          88                                         (sinks containing      35          85                                         14.2% ash)                                                                    ______________________________________                                    

EXAMPLE NO. 5

A Liddell seam coal with 9.3% ash yield, treated at 200° C., at a slurryconcentration of 29% with varying alkali concentration gave thefollowing % mineral removal:

    ______________________________________                                        Mole Ratio of NaOH:Ash                                                                        Slurry Conc.  % Mineral                                       (assuming ash is all SiO.sub.2)                                                               (% solids loading)                                                                          removed                                         ______________________________________                                        1:1 (5%)        29            50                                              2.5:1 (5%)      29            81                                              8:1 (5%)        29            87                                              ______________________________________                                    

The results show that to achieve significant mineral removal, thecaustic soda concentration should be greater than stoichiometric.

EXAMPLE NO. 6

A sample of Coal Cliff coal was processed with alkali over a range oftemperatures with subsequent treatment with acid and the ash levels weremeasured as follows:

    ______________________________________                                        Coal-Coal Cliff (20% ash db), Particle Size - -2mm,                           NaOH-15%                                                                      Temperature    Ash % (db)                                                     ______________________________________                                        150            4.6                                                            220            2.2                                                            260            2.3                                                            300            2.5                                                            ______________________________________                                    

The % mineral removal from a Piercefield seam coal in a 50% slurry atdifferent temperatures is shown below:

    ______________________________________                                         Temperature % Mineral Removal                                                ______________________________________                                        Piercefield Floats (2.6% ash)                                                 170          36                                                               200          64                                                               250          83                                                               Piercefield Rejects (12.9% ash)                                               170          63                                                               200          83                                                               250          90                                                               ______________________________________                                    

EXAMPLE NO. 7

Experiments carried out with a wide range of coals showed that theamount of organic matter dissolved varied considerably with rank, andincreased with temperature.

    ______________________________________                                                       % Coal Dissolved at the Various                                               Temperatures of Alkali Leaching                                Coal Name (Ash % db)                                                                           150°                                                                          220°                                                                            260°                                                                        300°                             ______________________________________                                        Dawson     8.3       Nil    Nil    Nil  Nil                                   Coal Cliff 20.0      Nil    Nil    Nil  Nil                                   Bowen      15.6      0.005  0.18   --   0.67                                  Huntley    23.8      Nil    Nil    Nil  Nil                                   Wongawilli                                                                    Great      22.0      --     0.47   --   1.68                                  Northern                                                                      Newvale                                                                       Cook       10.0      Nil    Nil    Nil  --                                    Ulan       17.6      Nil    0.30   0.50 --                                    Liddell    8.7       Nil    0.11   0.72 --                                    Blair Athol                                                                              8.2       0.07   1.07   --   --                                    Collie     4.6       0.75   --     --   --                                    Wandoan    8.2       2.10   --     --   --                                    Leigh Creek                                                                              13.6      3.97   --     --   --                                    Esperance  25.8      8.63   --     --   --                                    ______________________________________                                         Nil  color detected in solution but too small to measure accurately.          -- experiment not attempted.                                             

EXAMPLE NO. 8

The advantage of rapid heating and cooling is that there is less attackon the coal (i.e., as measured by the quantity of dissolved coal) andthe quantity of sodalite formed is less. A Liddell seam coal was heatedslowly up to 200° C. and cooled slowly over a period of 2 hours.Analyses for dissolved organics and ash content of alkalized coals werecompared with results for the same coal treated with rapid heating andcooling. The results indicate a marked improvement for the lattermethod.

    ______________________________________                                        Slow Heating and   Rapid Heating and                                          Cooling (2 hours)  Cooling (10 min.)                                          Dissolved Alkali Ash   Dissolved Alkali Ash                                   Organics  (sodalite)   Organics  (sodalite)                                   ______________________________________                                        Liddell Seam Coal (5.6% db), Particle Size - 200 μm                        1.30%     7.0          Nil*      4.2                                          Ulan Coal (17.6% db), Particle Size - 200 μm                               0.86%     11.6         Nil*      5.0                                          ______________________________________                                         Nil  too small to be collected on filter paper (but slight coloration of      liquor).                                                                 

EXAMPLE NO. 9

Ulan coal (17.6% db) washed to -2 mm was demineralized using alkali in atypical batch experiment at 220° C. peak temperature, following byacidification and washing. The product was separated into closely sizedfractions, and the percentage mineral removed was calculated for eachfraction from the ash yield. The following data were obtained showingsubstantially the same mineral matter for each fraction. Minorvariations occurred in the largest and smallest sizes because thelargest size contained some insufficiently dissolved quartz grains andthe smallest size contained a high proportion of iron formed byconcentration of fine hematite derived from the conversion of pyrites.

    ______________________________________                                        Particle Size Fraction                                                                          % Mineral Removed                                           ______________________________________                                        +1.4 mm           93.8   large quartz grains                                  -1.4 mm + 500 μm                                                                             96.3                                                        -500 μm + 425 μm                                                                          97.7                                                        -425 μm + 300 μm                                                                          97.1                                                        -300 μm + 250 μm                                                                          97.1                                                        -250 μm + 150 μm                                                                          97.3                                                        -150 μm + 75 μm                                                                           96.3                                                        -75 μm         94.4   high iron                                            ______________________________________                                    

EXAMPLE NO. 10

    ______________________________________                                        Coal - Liddell (8.6% db), Particle Size - 200 μm                           Liquor Analysis   Solid Analysis                                              Before Lime After Lime                                                                              Lime Filter Cake                                        mg/L              %                                                           ______________________________________                                        Si   2300       125       1.3                                                 Al   105        50        0.09                                                Fe   9.2        0.08      0.08                                                Ti   8.0        0.11      0.01                                                Ca   290        11.3      45.9 (excess lime)                                  Mg   0.15       0.47      0.47                                                Na   72.8 g/L   79.7 g/L  0.004                                               K    165        290       0.004                                               ______________________________________                                    

EXAMPLE NO. 11

To calculate the rate of lime reaction in regenerating the blackliquors, 350 g of Vaux seam coal and 1 L 16% NaOH autoclaved at 230° C.liquor filtered and limed 100 g.

    ______________________________________                                        Time      Si mg/L  Al mg/L   Na g/L  K mg/L                                   ______________________________________                                        0             2970     19.0    47.5    157                                    15   min.     560      4.5     46.3    190                                    30   min.     330      3.9     46.3    186                                    1    hour     195      4.5     46.3    172                                    2    hours    120      4.9     45.7    180                                    4    hours     85      5.2     46.3    186                                    6    hours     60      5.7     47.0    186                                    24   hours     55      6.0     47.0    172                                    ______________________________________                                    

EXAMPLE NO. 12

Sodalite concentrates can be collected in the fines under flow fractionfrom conventional countercurrent washing units.

Coal--Liddell (8.6% db), Particle Size -2 mm with 95% -1.4 mm +300 μm.

Sodalite Content of Fines---100 μm is 80.5% db.

EXAMPLE NO. 13

The quantity of sodalite on the alkalized coal can be removed byconventional froth flotation techniques, as shown below:

Coal--Ulan (12.6% db), Particle Size -2 mm.

Ash yield (sodalite concentration) of the alkalized coal=10.36% db.After froth flotation, the ash yield of treated coal floats=5.29% db.The separated sodalite appears in the flotation sinks fraction.

EXAMPLE NO. 14

Practically all acidified samples of silicic acid form gels if left toage. The most favorable conditions where gel formation takes a long timeare described below:

If the solutions are maintained at a pH of approximately 1 with acorresponding conductivity of between 10,000 and 50,000 μS(microsiemens), gel formation can be avoided. If the concentration ofdissolved salts increases the conductivity above 200,000 by adding moreacid or dissolved sodalite salts, then clear gels form slowly over a dayor so. Between 50,000-200,000 μS clear gels form over a week.

If the acid strength is a pH of 0.1 or lower and the quantity ofsodalite is high, then opaque gels form immediately. Again, if the pH isnear neutral, milky gels form with some precipitation and a liquid phaseis formed.

To obtain the optimum condition for prevention of gel formation in acoal sample, a general formula is as follows: If a coal contains between6-9% sodalite and is mixed with a quantity of water twice the weight ofcoal and maintained at a pH close to 1, then gel formation does notoccur within the time required to dissolve the sodalite and wash theacidified coal. If the sodalite concentration is twice as high or thequantity of water halved, then gel formation may occur in a day. (Idealconditions are pH=1 and conductivity=50,000 μs.)

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We claim:
 1. A process for the preparation of demineralized coal,comprising the steps of:(a) forming a slurry of coal particles with anaqueous alkali solution having an alkali content of from 5 to 30% byweight, such that said slurry has an alkali solution to coal ratio on aweight basis of at least 1:1; (b) maintaining said slurry at atemperature of from 150° to 300° C. for a period of from 2 to 20 minutessubstantially under autogenous hydrothermal pressure and rapidly coolingsaid slurry to a temperature of less than 100° C.; (c) separating theslurry into alkalized coal and a spent alkali leachant solution; (d)regenerating said alkali leachant solution for reuse in step (a) aboveby addition of calcium or magnesium oxide or hydroxide thereto toprecipitate minerals therefrom; (e) acidifying said alkalized coal bytreatment with an aqueous solution of sulfuric or sulfurous acid toyield a slurry having a pH of from 0.5 to 1.5 and a conductivity of from10,000 to 100,000 μs; (f) separating the slurry of step (e) intoacidified coal and a spent acid leachant solution; and (g) washing theacidified coal.
 2. A process as claimed in claim 1, wherein said slurryof coal and aqueous alkali solution has an alkali solution to coal ratioon a weight ratio of from 2:1 to 10:1.
 3. A process as claimed in claim1, wherein said alkali/coal slurry is maintained at a temperature offrom 170° to 230° C. for a time of from 5 to 10 minutes.
 4. A process asclaimed in claim 1, wherein the alkali/coal slurry is maintained at atemperature of from 170° to 230° C.
 5. A process as claimed in claim 1,wherein the alkali is selected from the group comprising sodiumhydroxide, potassium hydroxide and mixtures thereof.
 6. A process asclaimed, in claim 1, wherein the alkali/coal slurry is formed in acountercurrent reactor.
 7. A process as claimed in claim 1, wherein saidalkali solution has an alkali content of from 5 to 10% by weight.
 8. Aprocess as claimed in claim 1, wherein the alkali/coal slurry is held ata temperature of from 120° to 150° C. prior to being heated to, andmaintained at a temperature of from 170° to 230° in step (b).
 9. Aprocess as claimed in claim 8, wherein a physical separation step iscarried out between steps (c) and (e) to remove discrete particles ofsodalite and other reaction products of said alkali solution and saidcoal.
 10. A process as claimed in claim 1, wherein said alkalized coalis acidified by being introduced into an acid solution containingsufficient acid to produce the desire pH and conductivity stated in step(e).
 11. A process as claimed in said claim 1, wherein acidified coal iswashed with a solution of an organic acid and is subsequently washedwith deionized water.
 12. A demineralized coal obtained by a process asclaimed in claim
 1. 13. A process as claimed in claim 1, wherein atleast 50 wt. % of said coal particles used to form said slurry of step(a) have a particle size of at least 0.5 mm.